1. Field of the Invention
This invention relates to systems and methods for generating and propagating microwave to mm-wave electromagnetic radiation along a waveguide as a result of the nonlinear coupling of electron beam-driven electrostatic plasma waves within the waveguide.
2. Description of the Related Art
It would be highly desirable to be able to generate broadband, medium power (kilowatts) microwave to mm-wave radiation with a rapid frequency hopping and chirping capability over multiple octaves in frequency in a simple, low-cost and compact package. Keeping a device of this type light in weight would also be very important, since it would have various applications as a compact broadband transmitting mechanism for electronic warfare jamming applications. However, no devices have heretofore been developed that are capable of providing these functions in a satisfactory manner.
Various existing devices exist which might be considered for this application, but there are significant limitations to each. These include slow-wave devices such as travelling wave tubes, backward wave oscillators, magnetrons and Klystrons; fast-wave devices such as gyrotrons and free-electron lasers; and solid-state devices such as Gunn and IMPATT oscillators. The slow-wave devices produce too little mm-wave power, the fast-wave devices require very high voltages, high magnetic fields, and cannot be packaged compactly, while the solid-state devices provide narrow bandwidth and low power.
Another type of device, described in I. Alexeff and F. Dyer, Phys. Rev. Lett. 45, 351 (1980), is designated the orbitron maser. According to the authors, electrons are emitted from the inner surface of a cylinder by glow discharge, and are trapped in orbits about a thin wire which runs down the axis of a cylinder and has a positive voltage charge relative to the cylinder. The electrons drive a negative mass instability, which results in electron bunching. This in turn produces a space charge wave which couples to an electromagnetic waveguide mode. However, the orbitron maser requires highly fragile wire electrodes at mm-wave frequencies, and has too low an efficiency (in the order of about 10.sup.-6) for practical applications.
The injection of a powerful electron beam into a high-density plasma has previously been found to excite an electron plasma wave with a phase velocity less than the beam speed. The electron plasma wave is an electrostatic wave which oscillates at a frequency determined by the plasma density. The possibility of using the beam-plasma interaction to generate electromagnetic radiation was recognized when excitation of plasma waves by the two-stream instability was first discovered. However, the problem of coupling the RF energy out of the plasma prevented the development of practical sources or amplifiers based on this interaction. The coupling problem has its root in the fact that the RF energy is stored in an electron plasma wave which is purely electrostatic and trapped in the plasma. If the plasma is uniform, the electric field of each half-cycle of the wave accelerates the same number of electrons with alternating phase, so that no net source current is driven which can couple to an electromagnetic wave (electric field and density fluctuations are 90.degree. out of phase).
More recently, however, experimental observations and advancements in plasma theory have shown that physical mechanisms exist which permit the conversion of electrostatic waves to electromagnetic waves inside the plasma, and the direct radiation of these waves with the plasma acting as an antenna. These processes require that the electron plasma waves interact with a density gradient or other plasma waves in a nonlinear wave-wave interaction in order to conserve momentum. The latter interaction is often called three-wave mixing, since it involves the coupling of two electrostatic plasma waves to generate an electromagnetic wave. Such mechanisms were originally proposed to explain bursts of radio emission from solar flares. Evidence of plasma radiation due to these processes has been observed in the laboratory. However, no way to exploit this phenomenon in a practical device that extends to the mm-wave range, with a practical efficiency in excess of 10.sup.-4, has heretofore been devised.